This invention relates to improving the isolation between antennas located in close physical proximity where intercoupling between antennas has been a problem. More particularly, rather than seeking maximum separation of such antennas, in accordance with the invention the antennas (transmit and receive antennas, for example) are collocated as an antenna system with isolation enhanced by cancellation of coupled signals.
Where applications require multiple VHF radios and antennas in shipboard applications, previous approaches have relied upon the mounting of antennas at different locations on the ship chosen to provide the greatest possible physical separations. However, faced with limitations of ship dimensions, available mounting locations and multipath reflections from structural elements, the resulting multiple antenna systems have achieved poor, or, at best, moderate isolation between the antennas. These limitations of prior antenna arrangements are documented in Naval Research Laboratory Technical Memorandum 7550-401a, 11 Dec. 1985, which concludes that, for most ships, available antenna locations are such that it is anticipated that isolation only on the order of 20 to 40 dB can be achieved over the 30 to 88 MHz VHF range. Similar problems exist in other applications of land-based systems and systems for other frequency bands where large physical spacings between antenna locations are not practical or not possible.
It is, therefore, an object of the present invention to provide improved antenna systems which achieve high signal isolation between two antennas, without requiring isolated location of the antennas and, more particularly, such antenna systems able to achieve 50 to 60 dB isolation over a 30 to 88 MHz bandwidth in a shipboard installation environment.
It is a further object to provide high-isolation antenna systems, such as transmit/receive antenna systems, wherein separate transmit and receive antennas are collocated and arranged so that intercoupling effects between the antennas are cancelled to provide increased operational isolation.
It is also an object to provide dual antenna systems having omnidirectional coverage and supported on a single mast, wherein a first dipole type antenna may be supported on the mast above a second antenna that includes multiple dipoles arranged symmetrically around the mast, with high isolation achieved by the combination of the vertical separation between the antennas and cancellation of signals coupled between the antennas.